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Derivatives (Derivative Assets and Liabilities) (Details) - CAD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | $ 293 | $ 434
Derivative Asset, Collateral, Obligation to Return Cash, Offset | (126) | (181)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 491 | 491
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | (126) | (181)
Current Derivative Assets or Liabilities [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Total | 148 | 141
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Total | 260 | 227
Long term derivative assets or liabilities [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Total | 19 | 112
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Total | 105 | 83
Cash Flow Hedging [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0 | 7
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 5 | 7
Cash Flow Hedging [Member] | Power swaps [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0 | 5
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0 | 2
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0 | 2
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 5 | 5
Regulatory Deferral Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 105 | 195
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 7 | 28
Regulatory Deferral Hedge [Member] | Power swaps [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 2 | 5
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 1 | 3
Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 29 | 32
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0 | 4
Regulatory Deferral Hedge [Member] | Derivative instrument whose primary underlying risk is tied to commodity prices for coal purchases. |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 71 | 137
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 1 | 10
Regulatory Deferral Hedge [Member] | Natural gas purchases and sales [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 2 | 6
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 4 | 7
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 1 | 15
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 1 | 4
Held for Trading Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 187 | 230
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 479 | 456
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 62 | 125
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 76 | 162
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 125 | 105
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 403 | 294
Other Derivatives Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 1 | 2
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0 | 0
Other Derivatives Hedge [Member] | Interest Rate Swap [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 1 | 2
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0 | 0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Collateral, Obligation to Return Cash, Offset | (1) | (14)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | (1) | (14)
Designated as Hedging Instrument [Member] | Held for Trading Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Collateral, Obligation to Return Cash, Offset | (125) | (167)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | $ (125) | $ (167)

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